Heretofore, a variety of heat transfer printing methods have been known. Of these methods, a sublimation-type heat transfer printing method is now prevailing. In this method, a heat transfer sheet comprising as a coloring agent (printing agent) a sublimable dye which is retained by a substrate sheet such as paper is superposed on a heat transfer image-receiving sheet such as polyester woven cloth which can be dyed with the sublimable dye, and thermal energy is applied imagewise to the back surface of the heat transfer sheet to transfer the sublimable dye to the heat transfer image-receiving sheet, thereby producing an image on the image-receiving sheet.
Recently, there has been proposed a heat transfer printing method of the sublimation type, capable of producing various full-colored images on an image-receiving sheet such as a sheet of paper or a plastic film. In this method, a thermal head of a printer is employed as a heat application means, and a large number of dots in three or four colors are transferred to the image-receiving sheet in an extremely short heat application time. A full-colored image can thus be successfully reproduced on the image-receiving sheet.
The image thus obtained is very sharp and excellent in transparency because a dye is used as a coloring agent. Therefore, the heat transfer printing method of this type can produce an excellent half-tone image with continuous gradation, comparable to an image obtained by offset printing or gravure printing. Moreover, the quality of the image is as high as that of a full-colored photograph.
However, the image produced by the above heat transfer printing method is poor in both color density (image density) and resistance to light. This is a serious problem in this method.
To conduct high-speed printing, it is required that thermal energy be applied to the heat transfer sheet in an extremely short time of several seconds or less. However, both the sublimable dye contained in the heat transfer sheet and the heat transfer image-receiving sheet are not thoroughly heated within such a short heat application time. The resulting image, therefore, cannot have a sufficiently high density.
A sublimable dye which is excellent in sublimation ability has been developed in order to successfully conduct high-speed printing. In general, however, a highly sublimable dye has a low molecular weight. Fro this reason, an image produced on an image-receiving sheet using such a dye is poor in resistance to light, and the color of the image fades easily.
To solve the above problem, a sublimable dye having a relatively high molecular weight may be employed. Such a sublimable dye, however, cannot sublime instantly when heat is applied thereto, so that an image having a satisfactorily high density cannot be obtained by the above high-speed printing method.
Accordingly, an object of the present invention is to provide a dye and a heat transfer sheet for use in a heat transfer printing method employing a sublimable dye, capable of producing an image which has a sufficiently high density and is excellent in sharpness, fastness properties, and, in particular, resistance to light.